Hydride Transfer from Iron(II) Hydride Compounds to NAD(P)⁺ Analogues

Abstract

Iron­(II) hydride complexes of the “piano-stool” type, Cp*­(P-P)­FeH (P-P = dppe (<b>1H</b>), dppbz (<b>2H</b>), dppm (<b>3H</b>), dcpe (<b>4H</b>)) were examined as hydride donors in the reduction of <i>N</i>-benzylpyridinium and acridinium salts. Two pathways of hydride transfer, “single-step H^–” transfer to pyridinium and a “two-step (e^–/H^•)” transfer for acridinium reduction, were observed. When 1-benzylnicotinamide cation (BNA⁺) was used as an H^– acceptor, kinetic studies suggested that <b>BNA</b>^<b>+</b> was reduced at the C6 position, affording 1,6-BNAH, which can be converted to the more thermally stable 1,4-product. The rate constant <i>k</i> of H^– transfer was very sensitive to the bite angle of P–Fe–P in Cp*­(P-P)­FeH and ranged from 3.23 × 10^–3 M^–1 s^–1 for dppe to 1.74 × 10^–1 M^–1 s^–1 for dppm. The results obtained from reduction of a range of <i>N</i>-benzylpyridinium derivatives suggest that H^– transfer is more likely to be charge controlled. In the reduction of 10-methylacridinium ion (<b>Acr</b>^<b>+</b>), the reaction was initiated by an e^– transfer (ET) process and then followed by rapid disproportionation reactions to produce <b>Acr</b>_<b>2</b> dimer and release of H₂. To achieve H^• transfer after ET from [Cp*­(P-P)­FeH]⁺ to acridine radicals, the bulkier acridinium salt 9-phenyl-10-methylacridinium (<b>PhAcr</b>^<b>+</b>) was selected as an acceptor. More evidence for this “two-step (e^–/H^•)” process was derived from the characterization of <b>PhAcr^•</b> and [<b>4H</b>]^<b>+</b> radicals by EPR spectra and by the crystallographic structure confirmation of [<b>4H</b>]^<b>+</b>. Our mechanistic understanding of fundamental H^– transfer from iron­(II) hydrides to NAD⁺ analogues provides insight into establishing attractive bio-organometallic transformation cycles driven by iron catalysis